1. Field of the Invention
This invention relates generally to image postprocessing techniques and more particularly to a system and method using edge processing to remove blocking artifacts from decompressed images.
2. Description of the Background Art
Producing high-quality visual images using modern computer techniques is an important consideration of many computer manufacturers and designers. When displayed on a video monitor, an image frame typically comprises many separate picture elements or pixels which are each represented using a corresponding binary value. The computer system thus requires large amounts of digital information to represent each displayed image frame. To conserve memory space and expedite transmission of the digital information, modern computer systems typically code the image frames using a variety of compression techniques. One popular compression coding technique uses an encoder device to divide an image into a number of discrete blocks which are each processed and compressed independently. A corresponding decoder device subsequently decompresses the compressed image prior to display on a video monitor. Examples of conventional formats which utilize block-based image coding and decoding include JPEG, MPEG, H.261 and H263.
Referring now to FIG. 1, a block diagram of a sample blocked image 110 processed by a conventional block-based image decoder is shown. Sample blocked image 110 includes adjacent blocks 112 and 116 which are separated by boundary 114. Sample blocked image 110 contains sixteen blocks for reasons of clarity, however, in practice, a blocked image may typically contain a greater number of discrete blocks.
Blocking artifacts are relatively common to block-based encoder/decoder systems. Each discrete image block is processed and compressed separately, resulting in frequent variations in average pixel intensity between the various blocks. This causes the human eye to perceive the resultant image frame as a collection of individual blocks, as illustrated in FIG. 1.
For example, block 112 lies adjacent to block 116 along boundary 114. If block 112 and block 116 have different pixel intensities, the human eye will perceive an “edge” along boundary 114. This edge is created by the discontinuity in pixel intensity across boundary 114 between block 112 and block 116. Furthermore, this edge will have an edge intensity proportional to the magnitude of the average difference between the pixel intensity of block 112 and the pixel intensity of block 116.
One conventional postprocessing technique for reducing the block edges is low-pass filtering. The low-pass filter, however, smoothes both the block edges and the perceptually important features of the image, resulting in a blurred image. Adaptive filtering and image restoration techniques may also be used to reduce block edges, however, these techniques may also create new artifacts in the image. Therefore, in accordance with the present invention, an improved system and method is needed for using edge processing to remove blocking artifacts in image decoder devices.